Extreme ultraviolet light, e.g., electromagnetic radiation having wavelengths of around 50 nm or less (also sometimes referred to as soft x-rays), and including light at a wavelength of about 13.5 nm, can be used in photolithography processes to produce extremely small features in substrates, e.g., silicon wafers.
Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has an element, e.g., xenon, lithium or tin, with one or more emission line in the EUV range. In one such method, often termed laser produced plasma (“LPP”) the required plasma can be produced by irradiating a target material, such as a droplet, stream or cluster of material having the required line-emitting element, with a laser beam.
Heretofore, LPP systems have been disclosed in which each droplet is irradiated by a separate laser pulse to form a plasma from each droplet. Also, systems have been disclosed in which each droplet is sequentially illuminated by more than one light pulses. In some cases, each droplet may be exposed to a so-called “pre-pulse” and a so-called “main pulse”, however, it is to be appreciated that more than one pre-pulse may be used and more than one main pulse may be used and that the functions of the pre-pulse and main pulse may overlap to some extent. Typically, the pre-pulse(s) may function to expand the material and thereby increase the amount of material which interacts with the main pulse and the main-pulse may function to convert most or all of the material into a plasma and thereby produce an EUV light emission. However, it is to be appreciated that the functions of the pre-pulse and main pulse may overlap to some extent, e.g., the pre-pulse(s) may generate some plasma, etc. The increased material/pulse interaction may be due a larger cross-section of material exposed to the pulse, a greater penetration of the pulse into the material due to the material's decreased density, or both. Another benefit of pre-pulsing is that it may expand the target to the size of the focused pulse, allowing all of the pulse to participate. This may be especially beneficial if relatively small droplets are used as targets and the irradiating light cannot be focused to the size of the small droplet.
In addition to the above described techniques, U.S. Pat. No. 6,855,943 (hereinafter the '943 patent) which issued to Shields on Feb. 15, 2005 and is entitled “DROPLET TARGET DELIVERY METHOD FOR HIGH PULSE-RATE LASER-PLASMA EXTREME ULTRAVIOLET LIGHT SOURCE” discloses a technique in which only some of the droplets in a droplet stream, e.g., every third droplet, is irradiated to produce a pulsed EUV light output. As disclosed in the '943 patent, the nonparticipating droplets (so-called buffer droplets) advantageously shield the next participating droplet from the effects of the plasma generated at the irradiation site. Unfortunately, in some cases, these buffer droplets may reflect light back into the laser causing self-lasing, which among other things, can reduce the effectiveness of the laser's gain media in producing high energy pulses. This may be especially true for high gain (e.g., G=1000-10,000) infra-red lasers, e.g., CO2 lasers, which tend to self-lase rather easily.
A typically photolithography scanner exposes a portion of a moving wafer to a so-called “burst” of light pulses. In many cases, the pulse energy varies from pulse-to-pulse within the burst, and the accumulated energy of the burst, referred to generally as “dose”, is typically prescribed and must be controlled within a relatively small range. In addition to dose, some lithography operations also prescribe limits on the pulse-to-pulse energy variation within a burst. This is sometimes referred to as pulse-to-pulse energy stability. The lasers that are used in deep ultraviolet photolithography (DUV), e.g., excimer lasers at a wavelength of, e.g., 100-300 nm, typically have the ability to substantially vary pulse energy within a burst of pulses, and in some cases, on a pulse-to-pulse basis. This can be achieved, for example, by altering the discharge voltage used to create each pulse. This flexibility, however, is not available for all types of lasers. In some laser architectures, a continuously pumped active media may be used to generate laser pulses. For example, pulses may be generated in a continuously pumped oscillator using, for example, Q-switch or cavity dump mode, and then amplified by one or more continuously pumped power amplifiers. As used herein, the term continuously pumped laser devices (CW laser devices) refers to a laser device having a continuously pumped active media, e.g., continuously pumped oscillator and/or continuously pumped amplifier. Unlike the pulse—pumped laser devices described above, e.g., excimer discharge laser devices, the CW laser devices do not have the ability to quickly vary their discharge voltage, and as a consequence, are generally incapable of substantially varying their output pulse energy within a burst of pulses by altering their discharge voltage.
As indicated above, one technique to produce EUV light involves irradiating a target material droplet with one or more pre-pulse(s) followed by a main pulse. In this regard, CO2 lasers, and in particular, continuously pumped CO2 laser devices may present certain advantages as a drive laser producing “main” pulses in an LPP process. This may be especially true for certain targets, e.g., tin. For example, one advantage may include the ability to produce a relatively high conversion efficiency e.g., the ratio of output EUV in-band power to drive laser input power.
Along these lines, U.S. Pat. No. 6,973,164 which issued to Hartlove et al. on Dec. 6, 2005 and is entitled “LASER-PRODUCED PLASMA EUV LIGHT SOURCE WITH PRE-PULSE ENHANCEMENT” discloses that a variation in time delay between pre-pulse and main pulse in a Nd:YAG with Xenon targets LPP system results in a variation of output EUV intensity for time delays shorter than 160 ns.
With the above in mind, Applicants disclose a laser produced plasma EUV light source, and corresponding methods of use.